The recent increase in not only the public's interest in but dependance on the Internet and in other forms of telecommunication has led to an explosion of growth in optical technologies.
This growth has produced an unprecedented need for quicker, more efficient methods to produce not only optical fibers but also devices utilizing or based on such fibers. A cornerstone of this technology is the fiber Bragg grating, the basis for a majority of optical signal processing equipments. While producing Bragg gratings in optical fibers is well-known, the techniques for producing fiber cores that utilize these gratings for optical signal processing are labourious and slow.
Typically, an optical waveguide, such as an optical fiber, is exposed to ultra-violet (UV) light through a phase mask. The UV light produces a diffraction pattern that induces a change in the refractive index of selected sections of the fiber. The diffraction pattern produces bands on the fiber core that have a refractive index different from the core's normal refractive index. It is these bands, the spacing between them, length of the banded region, and the periodicity of the bands that allow optical signal processing. Through these bands, collectively known as gratings, optical filters, multiplexers, interferometers, and other processing devices are produced. Unfortunately, the amount of UV exposure determines the stability of the index change. Strong UV exposure produces stable gratings at a given operating temperature while weaker UV exposure produces unstable gratings at the given operating temperature. The unstable gratings tend to become weaker over time.
To overcome this undesirable quality, after exposure to UV light, fibers with gratings imprinted on them are typically annealed. Annealing a fiber involves exposing the fiber to high temperatures, typically by placing the fibers in an oven. The elevated temperatures eliminate the weaker, more unstable bands while the stronger bands survive and are rendered stable at some predetermined temperature. To obtain a specific band pattern or grating, the UV imprinting overexposes to produce a larger index change than what is required. Then, after the annealing, the resulting grating is measured to determine how much of the grating has been erased. Based on this measurement, new UV exposure conditions are calculated so that, after annealing, the desired grating is produced.
The above procedure is clearly unsatisfactory for larger scale manufacturing operations. Its trial and error nature is not only time consuming but also quite expensive in terms of manpower and effort.
Clearly, there is a need for a manufacturing process that avoids the costly and time intensive trial and error method.